Observation of change in the oxidation state at ferromagnet/insulator interface upon thermal annealing

It is reported that the magnetoresistance ( MR) of magnetic tunneling junct ion (MTJ) like Co1-xFex/Al2O3/Co1-xFex improves upon thermal annealing. We investigate the mechanism of this improvement by comparing the X-ray absorp tion spectra ( XAS) of half-MTJ structures (Co84Fe16/Al2O3) before and afte r annealing. Before annealing, XAS show the presence of few angstroms of Co - and Fe-oxides, which disappeared after annealing at 250 degreesC for 1/2 hour. We attribute enhanced MR upon annealing to the disappearance of Co an d Fe oxides at the interface which reduce the spin-polarization of the cond uction electrons and cause spin-flip scattering, both leading to inferior p erformance of MTJ.